When using the NextGEN gallery plugin you should check that none of your gallery posts or pages are incorrectly begin flagged as ‘do not index’ by WordPress automatically setting canonical URL’s.
It appears that on some NexGEN gallery sites WordPress is using the canonical setting on pages that are in fact unique and therefore should not have a canonical url set. For example, on my clients Japanese Woodlbock Prints site, pages that contain unique descriptions about the prints had canonical urls pointing back to the NextGEN galleries thumbnail page. As a result of this ‘misuse’ of canonical urls a significant part of the website was flagged non-indexable and therefore google would potentially not index those pages.
Looking at the screen capture below on the left it can be seen that SEO tools are reporting that a canonical url is being set for the page – even though the page is unique. The setting of the canonical url on that page is WordPress’s default handling but in this case it is wrong. The screen capture on the right shows the same page with no canonical url present which for this site is the correct setup.
Canonical urls were turned off on the site simply by editing the default-filters.php file to comment out the
add_action( 'wp_head', 'rel_canonical' ); line.
Some SEO plugins allow you to set or unset canonical urls so if your using one then you’ll need to check its settings. I used All in One SEO on the gallery site with Canonical URL’s unselected in its options. Another very popular SEO plugin is WordPress SEO, it however has no setting options for canonical URL’s. It just forces them onto the user even if you edit the default-filters.php file as suggested above. We were hoping to use WordPress SEO but because of its canonical handling (or more correctly, lack of) we couldn’t use it on the NextGEN gallery site. See WordPress SEO Plugin for a possible solution to this issue – we’ve not tested it so I’d recommend you hunt down the WordPress SEO support pages on wordpress.org to see if there’s been any feedback on it.
If you are a NextGEN user I recommend that you check to see that there is no inappropriate setting of canonical urls on your content pages. Whether canonical should be turned off on a site depends on it’s impact on an individual site. For some it may be an issue and for some it may not be.